Method of arranging all the parts (7)
I believe the answer is:
casting
'method of arranging' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'all the parts' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
The remaining letters 'casing' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for casting that I've seen before include "Shaping in mould; assigning actors to" , "-- Vote; -- couch" , "Choosing actors" , "Throwing" , "shell across back of fort" .)